1. Field of the Invention
This invention relates to an optical low-pass filter and more particularly to an optical low-pass filter suited for a color video camera which uses a stripe filter or a color mosaic filter.
2. Description of the Prior Art
A conventional low-pass filter of this kind uses a double refractive crystal such as quartz as disclosed in a Japanese utility model publication No. SHO 47-18688.
With the low-pass filter using a double refractive crystal, an image can be divided into two images of ordinary rays and extraordinary rays and, by controlling the extent of discordance between the two images, OTF can be reduced to almost zero with respect to a desired wave length and a desired spatial frequency. However, since quartz is expensive, a less expensive low-pass filter is desired.
To meet this requirement, various low-pass filters have been proposed including a low-pass filter of a multi-planar prismatic shape disclosed in U.S. Pat. No. 3,716,666; and low-pass filters of the diffraction grating type as disclosed in Japanese patent publication No. SHO 49-20105 and laid-open Japanese patent application Nos. SHO 48-53741, SHO 53-119063 and SHO 55-48724. The present invention is directed to the preparation of a low-pass filter of the type using a diffraction grating.